With a surface reflectance sensing system, a material to be tested is illuminated by a light source, and reflected light is measured with a light sensor. The light can be monochromatic, a range of wavelengths, or a mixture of wavelengths, such as ‘white’ light. This can be accomplished either by filtering the emitted light or the sensed light.
For example, the light source can be a wide-band tungsten-halogen or a tungsten-deuterium light that is filtered. The filtered light is transmitted to the surface, and then reflected and carried back to a sensor via an optical fiber. The wavelengths can be separated by a slit and diffraction grating, and measured by a photodiode array or CCD array and an analog-to-digital (A/D) converter. Depending on a width of the slit and a fineness of the diffraction grating, a resolution of 1 to 4 nanometers can be attained across a range of 350 to 1100 nanometers. There, the A/D converter is the most expensive component of the system.
In another surface reflectance system, narrow-band LEDs are used to emit light at known wavelengths, and photodiodes and an A/D circuit are used to measure the reflected light. Although, that type of system is less expensive, the small number of available colors available, e.g., five, makes it less suitable for analytical work.
LED-based systems have been used for color matching in the graphic arts, printing, photocopying, publishing, and paint-matching trades.
All of the prior art systems use conventional A/D circuits. In such circuits, the phototransistor or photodiode is a light-controlled current source or light-controlled resistor. The current generated by the photodiode device is amplified and directly measured by the A/D converter. Because the directly measured instantaneous current is typically only a few microamperes, a sensitive, low-noise amplifier must be used. That substantially increases the expense, size and power requirements of the system.
In addition, silicon photodiodes have a peak sensitivity at roughly 670 to 700 nanometers, extending into deep infrared, and are relatively insensitive to blue and ultraviolet light. Therefore, a proportional filter is used to ‘flatten’ the response to match that of the human eye, which decreases the sensitivity of the system. Furthermore, the dynamic range of a conventional 16-bit converter is roughly 64,000:1, with a considerable power consumption.
Therefore, there is a need for surface reflectance sensors and spectrophotometers that overcome the problems of the prior art.